I think you've moved on in your plans but my comment about modifying the Grainfather sparge water heater is that I found the walls to be a little bit thin so holding much weight was a challenge without leaks. I punched three holes ( +1 for the Q Max by the way ), being a water inlet up top, a thermowell and a tap down at the bottom. I found that a standard two piece tap was a bit of a balance to fit without a leak, and adding tubing or a pump just led to leaks. So I now use a mini 1/2" BSP fitting. Of course it's very possible that I was doing something wrong.